Focusing device for electron beams



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Aug 25, 1964 F. H. sAMUELsoN FOCUSING DEVICE FOR ELECTRON BEAMS 2 Sheets-Sheet 1 Filed March 29, 1962 FRED H. SAMUE @K M 6k o Aug 25, 1954 F. H. sAMuELsoN 34535 FOCUSING DEVICE FOR ELECTRON BEMS Filed March 29, 1962 2 Sheets-Sheet 2 /NVEN TOR FRED H. SAMUELSON By M @Urn //ENT United States Patent O 3,146,335 FOCUSING DEVICE FOR ELECTRON BEAMS Fred H. Samuelson, Thompsonville, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Mar. 29, 1962, Ser. No. 183,651 6 Claims. (Cl. 219--121) My invention relates to working materials with an intense beam of charged particles. More particularly, my invention relates to a method and apparatus for controlling the focusing of an electron beam in an apparatus which uses such beam to machine or perform other operations on any material.

Electron beam machines, as they are generally known, are devices which use the kinetic energy of an electron beam to work a material. U.S. Patent No. 2,793,281, issued May 21, 1957, to K. H. Steigerwald, discloses such a machine. These machines operate by generating a highly focused beam of electrons. The electron beam is a welding, cutting and machining tool which has practically no mass but has high kinetic energy because of the extremely high velocity imparted to the electrons. Transfer of this kinetic energy to the lattice electrons of the work piece generates higher lattice vibrations which cause an increase in the temperature within the impingement area sufficient to accomplish work.

It is a fact that a beam of highest power densitypower per unit if impingement area-is more effective. That is, a high power density beam can accomplish the required work in the shortest possible time and thus minimize heat conductivity so that the material adjacent to the area being worked is relatively unaffected. In order to obtain high power density, precise electron optics have to be applied in focusing the beam. Beam power density is defined as:

where =beam current V=electron accelerating voltage A=area of beam impingement on work piece From the above discussion and Equation 1 it becomes apparent that optimum conditions for working material with an electron beam require the smallest possible spot size consistent with the type of electron optics used. It should be noted that the electron optical system employed is modified in accordance with the type of operation to be performed. That is, the minimum spot size obtainable in, for example, an electron beam Welder would be too large for a cutting operation.

The basic formula for the electron beam current in an electron optical system has been derived by Langmuir and is set forth on page 750 of Electron Optics and the Electron Microscope, published by John Wiley and Sons Incorporated, New York, in 1957, and reads:

(2) i=C0Vd8f3 where:

Co=depends only on the electron optics and filament heating d=diameter of beam impingement on work piece Solving this equation for d2 and combining with 1rd2 A-T the beam impingement area is found to be:

7r i 3/4 (3) 1:40am

"ice

This equation shows that the spot size and thus the beam power density is dependent on acceleration voltage, beam current and the electron optics of the machine.

To obtain maximum focusing, i.e., smallest obtainable spot size, prior art methods used a trial and error approach. That is, prior to my invention, a target such as a tungsten disc was placed under the electron beam at the same height at which work was to be performed. The beam was then allowed to impinge on this target while the spot size was observed visually through an optical system comprising a microscope. This method has several inherent disadvantages. First, as the beam approaches focus, the beam power density increases and the beam begins to destroy the surface of the target. This tends to prevent accurate focusing. Secondly, when an irregularly shaped piece was to be worked, this time consuming operation had to be repeated at each different height of the work piece.

My invention overcomes the above disadvantages by providing a novel method and apparatus for focusing an intense beam of charged particles.

It is therefore an object of my invention to focus a beam of charged particles.

It is another object of my invention to focus a beam of charged particles rapidly and accurately.

It is yet another object of my invention to automatically focus a beam of charged particles.

It is also an object of my invention to focus a beam of charged particles by utilizing a measurement of the energy in the defocused beam.

It is still another object of my invention to sense the energy in a defocused beam of charged particles in order to determine the focus of the beam.

It is another object of my invention to obtain an accurate and repeatable degree of defocus of a beam of charged particles.

These and other objects of my invention are accomplished by positioning a sensing element adjacent to the spot where work is to be performed by a focused beam of charged particles and at the same height as the piece that 4is to be worked. The area or spot size of the unfocused beam will be relatively large. As can be seen from equation 1 above, large spot size means low beam power density. Therefore, impingement of the unfocused beam on the sensing element will not damage the element. Since the sensing element is positioned adjacent the spot to be worked by the focused beam, as the beam is brought into focus fewer particles will impinge upon the sensing element. By connecting a resistance element between the sensing element or ground, the number of particles impinging on the sensing element can be measured as a voltage developed across the resistance element. The magnitude of this voltage gives an indication of the degree of focusing of the beam. That is, as the beam is brought into focus the spot size decreases and fewer of the particles in the beam will impinge upon the sensing element. The voltage across the resistance element, therefore, decreases as the beam is brought into focus. By causing my invention, a beam of charged particles can be rapidly and accurately brought to focus at a desired point manually or automatically with appropriate circuitry.

My invention may be better understood with reference to the accompanying drawing in which:

FIGURE 1 is a schematic view of an apparatus capable of being employed in the practice of my invention.

FIGURE 2 is an enlarged view of a sensing element which may be used with the apparatus of FIGURE 1.

FIGURES 3 through 5 are illustrations of the phenomenon which occurs as the beam is focused.

In FIGURE 1, an electron beam machine is indicated generally as 10. The machine comprises a Vacuum chamber 12 containing a work piece 14 positioned on a movable table 16. The machine also comprises an electron beam column 18 containing a source 0f electrons, beam forming means and beam focusing means. The source of electrons comprises a directly heated cathode or filament 20 which is supplied with heating current from filament voltage supply 22. An acceleration voltage is supplied to the cathode 20 from a high voltage supply 24 through an acceleration voltage control 26 which allows for adjustment of the acceleration voltage. An apertured anode 28 is positioned in the electron beam column 18 between the cathode and the work piece. The anode is connected to the case of the machine which is grounded at 30. The difference in potential between the cathode 20 and the anode 28 causes the electrons emitted from the cathode to be accelerated down column 18. The electrons are focused into a beam, indicated generally at 50, by an electron optical system comprising adjustment coils 32 and 34 and magnetic lens 36.

Under the operating conditions, the beam impinges on work piece 14 where it gives up kinetic energy in the form of heat. The work piece 14 may be moved beneath the beam by movable table 16 and the beam may be detiected over the work piece by means of deliection coils 38. Positioned adjacent cathode 20 is a control electrode 4i). This electrode may be of the type disclosed in U.S. Patent No. 2,771,568, issued November 20, 1956, to K. H. Steigerwald. This control electrode is normally maintained at a voltage which is more negative than the voltage applied to the cathode. The magnitude of this bias or voltage difference is variable by adjusting bias voltage control 42. The control electrode while aiding in the focusing of the beam, also performs the same function as the grid in an ordinary triode vacuum tube.

When it is desired to focus the electron beam so as to perform work upon a work piece such as that shown at 14, a sensing element 90 is positioned at the same height as the work piece and adjacent to the spot where the operation on the work piece is to be accomplished. The sensing element 90 is positioned manually by any means 92 known in art. At this instant, the table 16 which carries the work piece has been moved to one side. The Sensing element 90 is shown in FIGURE 2 and consists of a semi-circular conducting element 94 supported by insulator element 96. The sensing element shown in FIG- URE 2 is, of course, not shown to scale. Actually, the conducting element 94 could be merely a thin foil or even a metallic coating deposited on insulator 96. By connecting a resistor 44 to the sensing element by iiexible lead 46, the current through the sensing element caused by the impingement of the unfocused beam thereon may be sensed. As the current ows through resistor 44 to ground, a voltage will be developed across resistor 44. A voltmeter 48 is connected in parallel with resistor 44 to give a visual indication of the voltage developed across the resistor.

After machine has been energized and steady state conditions achieved, the beam may be found to initially be focused below or above the spot where work is to be performed as shown in FIGURES 4 and 5, respectively. Under either of these conditions, a number of the particles in the-beam will impinge upon the sensing element, as shown in FIGURES 4 and 5, thereby causing a voltage to be developed across resistor 44. As the focus of the beam is adjusted, for example, by adjusting the current supplied to the magnetic lens by adjustable current lens supply 52, the voltage across resistor 44 will either decrease or increase depending on whether the beam is being adjusted away from or toward being focused at the desired spot. As the beam is brought closer to focus, fewer particles will impinge upon the sensing element and the voltage across resistor 44 will accordingly decrease. At the condition of focus, as shown in FIGURE 3, only stray electrons should impinge upon the sensing element and the voltage across resistor 44 Will be at its minimum value. Therefore, by observing voltmeter 48 while manually adjusting the lens current supply 52, the beam may be brought to focus at the desired height by dipping That is, a minimum reading on meter 48 corresponds to that lens current which will cause the beam to be focussed at the point corresponding to the height of the work piece.

My invention may be automated by use of a circuitry, such as that to be described below, for seeking and holding the minimum obtainable voltage across resistor 44 by automatically adjusting the focus of the beam. In describing this circuitry, it should be noted that the wave forms appearing at various points throughout the circuit are shown by solid-line waves adjacent the corresponding vpoints at which they appear in the drawing. The voltage across resistor 44 may be applied to D.C. amplifier 54. The output signal from this amplifier, as shown by the wave form above conductor 56, has the same shape but is of opposite polarity to the voltage which is developed across resistor 44. When it is desired to automatically bring the beam into focus, a switch S-1 is manually closed to the lower contacts as shown. The voltage on conductor 56 is, therefore, applied to the solenoid of a relay Ry-l. The gain of amplifier 54 is such that the minimum voltage output of the amplifier, which occurs at the condition of focus, is still of sufficient magnitude to cause enough current to ow through the solenoid of relay Ry-l to cause closing of the contacts K1 and K2 of this relay. The closing of these contacts permits current to liow from source 58 through the solenoid of relay Ry-2 to ground. Current flow through the solenoid 0f relay Ry-Z causes the closing of contacts K3 and K4 thereby connecting A.C. voltage source 60 to servomotor 62 via conductor 64. The shaft 66 of motor 62 is connected to adjustable lens current supply 52. Closing of contacts K3 and K4 energizes motor 62 which in turn causes the output of lens current supply 52 to be varied. As the lens current supply 52 is driven by motor 62, the voltage across resistor 44 decreases as the current to the lens approaches that value which will focus the beam at work piece 14. As this condition of focus is approached, the voltage across resistor 44 and hence the output voltage vfrom amplifier 54 approaches its minimum value.

The output of amplifier 54 is also applied to a differentiator 68 to produce the wave form shown, which is a signal corresponding to the rate of change of the voltage across resistor 44. The output voltage from differentiator 68 is applied to a limiter 70 which removes all positive going potentials. This limiter may, for example, be a double diode limiter such as is shown at page 125 of Electron-Tube Circuitry, by Seely, published by McGraw- Hill, 1950. The output of limiter 70 is applied to a monostable multivibrator 72 and also via conductor 74 to an addition circuit 76. The leading edge of the output wave from limiter 70 triggers multivibrator 72 which produces a square wave output voltage having a relatively long pulse width. This square wave is also applied to adder 76 where it is added to the output of limiter 70 to produce lthe wave form shown above conductor 78. The magnitude of the square wave voltage is chosen so that it will cause the diode to begin to conduct. When diode 80 conducts, current will flow through the solenoid of relay Ry-S, conductor 78 and resistor 82 to ground. The plate voltage which will cause diode 80 to begin conducting may be varied by adjusting the negative bias on the cathode of diode 80 by varying the setting of the potentiometer 84.

As can be seen from the wave form developed across resistor 44, during focusing, as the point of focus is approached, the voltage across the resistor decreases. When the point of focus is passed, the voltage across the resistor begins to increase. Therefore, when the voltage across resistor 44 is differentiated, a wave form is produced which crosses the Zero axis at the point of minimum voltage which is the point of focus. Limiting of the differentiated voltage results in a signal which increases sharply, in a negative direction, and then decreases to zero at the point of focus. By adding this negative voltage to the positive square wave voltage output of the multivibrator, a signal is generated which becomes equal to the magnitude of the square wave Voltage at the instant when the limiter output becomes Zero. This is, of course, the instant when the voltage across resistor 44 is at its minimum value which is the point of focus. Therefore, when the voltage from limiter 70 decreases to Zero, diode 80 will conduct causing current to flow through the solenoid of relay Ry-3. Current ow through the solenoid of Ry-3 causes switch S1 to be opened. The opening of switch S-1 deenergizes the solenoid of relay Ry-l thereby opening contacts K1 and K2 and isolating current source 58 from solenoid of relay Ry-Z. Deenergization of relay Ry-2 in turn causes opening of contacts K3 and K4 which isolates motor 62 from source 60 thereby causing the lens current supply to stop scanning at the point of focus.

Once the focus condition has been achieved, the beam may be maintained in focus by observing meter 4S and compensating for any increase in the reading of the meter by manually adjusting lens current supply 52.

While the preferred employment of my invention is shown and disclosed, various modifications and substitutions may be made without deviating from the scope and spirit of my invention. For example, various other circuits for automatically focusing the beam might be utilized. Also, while I have disclosed my invention in terms of varying the focusing by adjusting the lens current supply, various other parameters might be adjusted either singly or along with the lens current supply. Referring back to Equation 3, it is obvious that either beam current or acceleration voltage might be varied to focus the beam without departing from the teaching of my invention. In this respect, beam current might be varied by adjusting the control electrode voltage or the filament temperature. It is also within the scope of my invention to maintain the beam focused at a particular spot, locate this spot by manipulation of the sensor and provide means to raise and lower the table upon which the work piece rests. Similarly, for large work pieces, it is feasible to employ a telescoping electron beam column and thereby raise and lower the electron gun while maintaining the work piece at the same vertical level. In this connection, it may be desired to work with a defocused beam. For example, it may be necessary to Weld with a machine having an electron optical system suitable only for cutting. Under these circumstances, my invention may be used to obtain an accurate focus at the target and thereafter this focus point and the work piece can be moved relative to one another, such as by raising the work piece or electron gun, to provide a repeatable degree of defocus.

Thus, my invention has been described by way of illustration rather than limitation and accordingly it is understood that my invention is to be limited only by the appended claims taken in view of the prior art.

6 I claim: 1. An electron beam welding and cutting machine comprising:

means for producing a beam of electrons,

means for varying the focus of said beam,

means for positioning a work piece in line with said beam,

a sensing element positionable at the same level as and adjacent to the spot Where work is to be performed,

means associated with said sensing element for measuring the energy in fthe defocused beam that impinges on said sensing element,

means responsive to said measuring means for causing adjustment of said means for varying the focus of the beam.

2. The apparatus of claim 1 wherein the sensing element comprises a conducting member shaped so as to extend at least part way around the focused beam.

3. A method of working materials with an intense beam of charged particles comprising:

generating a beam of charged particles,

positioning a sensing element at the level at which a material is to be worked and adjacent the spot where work is to be performed,

measuring the energy in the defocused beam which impinges on the sensing element,

varying the focus of said beam to achieve a minimum energy measurement, and

removing the sensing element from the area adjacent said beam and thereafter positioning a work piece in line with said beam.

4. The method of claim 3 wherein the charged particles are electrons.

5 Apparatus for sensing the focusing of an intense beam of charged particles comprising:

a sensing element positionable along the axis of and adjacent to the desired focal point of an intense beam of charged particles,

means connected to said sensing element and responsive to the impingement of charged particles in the initially defocussed beam thereon for generating an electrical signal commensurate with the number of charged particles impinging on the sensing means, and

means connected to said signal generating means and responsive to said signal commensurate with the number of charged particles impinging on said sensing means for providing an indication of the focussing of the beam whereby the minimum obtainable indication corresponds to the focussing of the beam at a point adjacent to said sensing element.

6. The apparatus of claim 5 wherein the sensing element comprises a conducting member shaped so as to extend at least part way around the focused beam.

References Cited in the le of this patent German application 1,072,763, printed Jan. 7, 1960, Kl.21h. 

1. AN ELECTRON BEAM WELDING AND CUTTING MACHINE COMPRISING: MEANS FOR PRODUCING A BEAM OF ELECTRONS, MEANS FOR VARYING THE FOCUS OF SAID BEAM, MEANS FOR POSITIONING A WORK PIECE IN LINE WITH SAID BEAM, 